UNIVERSITY OF CAMBRIDGE DEPARTMENT OF PHYSICS . Tk?LBPHOSE (~AURHIDCE 55478 CAVENDISH LABoRATORY ' FREE SCHOOL LANE . CAMBRIDGE Be2r Xi s s Frank1 in, * . . f , \ I ::-:I?L returnin~~ the tw papers whi'ch you so kindly lent to Jim, which as you earn $agine I read with very great interest. !: X:I sorry not to have sext them before, but Jim and I had to write FT. pper before he left for the States, ar;d this kept us very busy. I 212 enclosin?; a feyi torments on the Da-52 rs on salt,on Riley al?,d Gster's results,- ahd and some longer rexarks factors. a problem it is!. on calcu7.ating strticture ;:.hq f ..L L" : With best wishks, , Yours sincemly, ,. I?. f-I. C. `Crick. p . 7 p . 9 p.12 p.11 2.11 I ar;? not really clear about fibres T!:lhi&h only.give Structure: ti. I";OV! d.0' the xater content, - .. \ intensities ard the equatorial spcin,~ :::-srqe vith different x,3:,? ?OFi~Z?l?iS on v;ith Rile;;' ', OBter. 'I take it this is . Colurm 4 c& their Teb?.~ 1, 0~1 ,is it Colur~ 5? I agree that the phosphates rmst be accessible; The . . general evideme Suggesting they abe on the "o-tieside" seem plausible, but less corq:elling. I had i?ot realised that calf thyrms i>:as the 'only material to give Structure A. Ia this still true? Is it p?hap. due to the sugeriop method of'preparation used for calf thyrms? ', I am not quite 'clear how one cati be certain that the unit cell is truly face-centered monoclinic, and not i?eally . , . triclinic, with txo angles 90'. The 13oin-L ,is iq0rtxn-t because if the unit cell is strictly C 2 one rmst hscve the $ DEA chains i-n pairs, running in opposite directions. I Qke it that "12.14 .Et' is a slis. Jerry Donohue is worried because in Fig. 3 the::e is so little yector density at x - 40 a, z = o. A Mope mtural choice would have 'oeen x = 38, z = o. As you do got list your observed equatorial spacin;;s, it is difficult to see how thjrgs would fit (Incidentally 22 x 3 = 38).. Is the effect perhaps due to the negative contributions from the . . e p.15 'As you know,' we have never believed the ante-helical . . implications of the last paragraph, because of polyI:enzyl , glutamte. Would. you call an,ll-fold axis a high de;greeI : . of symmetry? ' Jim tells me that you claimed that "the v:ater had only 3. '-I neneral lowering effect, and that the G Za+ was neglizible't. Thi s is not strictly correct. For calculating structure factors for the lo-qer spcings it is convenient to qilow for the water by ta%inz -I p* g average electron density of vlater as the "zero" of electron density, Thus any group which has the same electron density as water makes no contribution. For each group its average eiectron density must be cai culated, and that of water subtracted, before its contribution can be given its proper weight. Thus, if the electron density of a base were , say, 1.2 times that of water, only a fraction ( $$) of its electrons would be counted. On the other hand the Na+ ion, due to ' strong electrostrictive , probably has a very small (or ne&tive?j partial specific volume, and thus almost.all of its electrons VI:z-iil coiint. Thus the effect of one I':&+ w be about the same as that of a base. It is-this necessity for allowing for the viater v:hich makes structure factor calculations difficult. Riley and Oster There seems little doubt that at least some of R. and 0 ' s long spacings are genuine, and it is a pity that in studying Struct&e i? the bc?lckstop in your experiment MS placed so that it would hide any such, sI;acings; though one can see you wanted short exposures. It is interesting to note that the spacings in your Faper 1 , plate 6; are almost exactly in the ratio of 3ibO*and b&$0 suggested by R. and 0. ' If: it were not for this 'complication of a longer spacing, one could use their results to obtain the number of chains in Structure E. I~Ioreover the chaulzes of the intensities of the equatorial reflexions as the spacings i.ncrease ~ou7.d be very inf 3r:mtive. I don't feel much progress ca.n. be made with Structure B until the long-spscin,~ Fosition is cleared up. 1 surmise the micelie structure is caused by the 10 -fold screw: axis of the fibre trying to give a hexagonal pack and not. quite making it. *I notice that your plate 5, raper 1, shoi7.s "s~~otsSr'. Are these due, to salt in the specimen, or to some other cause? Cster and :?iley had a iSaG ring in one'of their speciments, and also sot a sharp 3.25 A reflexion which looks fishy to me. The reason I ask is 8 s 331 +-VT . The equatorial spacings for Structure E a?rear to sho:;t one main spot (neglecting doubling for the moment) . If the structure is pseudo-hexagonal this is likely to be eith.er ~4 C + or -&w. If a++, it means a great lump at the origin of the C projection. This could only be phosphrntes near the centre. If w 4, it ceans a lot of material bet;cJeen the hel'ices (unless your --- then I a,m not clear if it ~:ould give the right ansq,E:er) ~ Part of this interhelical material could be the Xa+, but t'nis seems hardly enou,rh. ZoT:rever, if some extra salt were there this ai,cyht Pe .C.. sufficient. Koreover , it v~ould explain a.,lot of t.he ~uz;sZi:~~ d.ens ity discrepencies which are dif.ficult to acc.y*-t `*`:.. a `,: - zodel . Of course, you add distilled -<;atep yG:I:;e;~ -v:-z -. -. t,i-,c ~+-b~~~ p l:!C;r:**L3 - but is it quite certain that, in the specimens which give X-ray @otos, you have o_rlly the same amount of Ua+ (or other ions) as E0.c ? L The diierma is a very re,al one because in` effect one i.s deducing on.? "radius,!' from the zenera rcf'lexions and another. from t.he equatorial s `. -2- .' .